1. Field of the Invention
The present invention relates to a non-reduced dielectric composition, which may be used for dielectric materials, such as monolithic ceramic capacitors, using base metals such as nickel as an internal electrode material.
2. Description of the Related Art
Conventional dielectric ceramic materials containing BaTiO.sub.3 as a main component exhibit semiconductive properties when they are reduced as a result of firing under a neutral or reducing low oxygen partial pressure. Therefore, materials which do not melt at the high temperature at which the dielectric ceramic material is sintered, and which do not oxidize when fired under a high oxygen partial pressure and change the dielectric ceramic material to the semiconductor, for example noble metals such as palladium and platinum, must be used for the internal electrodes. Such problems significantly have hampered the low-cost production of the monolithic ceramic capacitors.
In order to solve the above problem, the use of base metals such as nickel, for example, has been required in internal electrode materials. When such base metals are used as internal electrode materials and fired under conventional conditions, the electrode materials become oxidized and do not accomplish the desired function as electrodes. For use with such base metals as internal electrode materials a dielectric ceramic material has been required which does not cause a change into a semiconductor state even when fired under a neutral or reducing low oxygen partial pressure, and which has the sufficient specific resistance and excellent dielectric characteristics of a dielectric material for a capacitor.
As dielectric ceramic materials satisfying the above conditions, for example, a BaTiO.sub.3 --CaZrO.sub.3 --MnO--MgO composition in JP-A-62-256422, a BaTiO.sub.3 --MnO--MgO-rare earth metal oxide composition in JP-A-63-103861, and a BaTiO.sub.3 --(Mg, Zn, Sr, Ca)O+Li.sub.2 O--SiO.sub.2 --MO (wherein MO represents BaO, SrO, or CaO) in JP-B-61-14610 are proposed.
However, the non-reduced dielectric ceramic composition disclosed in JP-A-62-256422 tends to form different phases including CaZrO.sub.3 and CaTiO.sub.3 during firing step, and Mn.
In the non-reduced dielectric ceramic composition disclosed in JP-A-63-103861, the dependency of the insulation resistance and capacitance on temperature are significantly affected by the particle size of the main component, BaTiO.sub.3, so that the process of obtaining stable characteristics is hard to control. Moreover, the product (CR) of the insulation resistance and electrostatic capacitance ranges from 1,000 to 2,000 (.OMEGA..F).
Further, in the composition disclosed in JP-B-61-14610, the dielectric constant of the resulting dielectric ceramic ranges from 2,000 to 2,800, which is lower than that of conventional ceramic compositions using noble metals such as palladium, i.e., 3,000 to 3,500. Thus, the substitution of this composition for conventional materials, for the purpose of the cost reduction, is not favorable to the miniaturization of a capacitor with higher capacitance.
Although all of the non-reduced dielectric ceramic compositions, including the above-mentioned compositions, which have been proposed recently have higher insulation resistances at room temperature compared with conventional materials, the resistance tends to drastically decrease at a higher temperature. Therefore, the resistance against high temperature is low, and that causes difficulty in thinning of the dielectric substance, so no thin multilayer capacitor using a non-reduced dielectric ceramic composition has been realized. Further, conventional non-reduced dielectric ceramic compositions exhibit a low resistance against high humidity in comparison with conventional materials using palladium as the internal electrode.
The present inventors have proposed novel non-reduced dielectric ceramic compositions in JP-A-5-09066, JP-A-5-09067, and JP-A-5-09068 in order to solve the above problems. Demands for compositions having further excellent properties have increased depending on properties required in the market, in particular, at a high temperature and high humidity.